Add In-Reply-To header to notification emails

We add a Message-ID header to the notification email about new ticket,
the value of ticket's mailing list is used. For other notifications, we
insert an In-Reply-To header with the same value. This implements "flat
replies" as described in https://todo.sr.ht/~sircmpwn/todo.sr.ht/97.

Closes https://todo.sr.ht/~sircmpwn/todo.sr.ht/97

tests/test_assignment.py

@@ -25,6 +25,9 @@ def test_assignment(mailbox):
     assert mailbox[0].subject == "~foo/bar#1: Hilfe!"
     assert mailbox[0].body.startswith(
         "You were assigned to ~foo/bar#1 by ~assigner")
+    assert mailbox[0].headers["In-Reply-To"] == (
+        f'<~{tracker.owner.username}/{tracker.name}/{ticket.scoped_id}@example.org>'
+    )
 
     # Assignment is idempotent
     assign(ticket, assignee1, assigner)
@@ -40,6 +43,9 @@ def test_assignment(mailbox):
     assert mailbox[1].subject == "~foo/bar#1: Hilfe!"
     assert mailbox[1].body.startswith(
         "You were assigned to ~foo/bar#1 by ~assigner")
+    assert mailbox[1].headers["In-Reply-To"] == (
+        f'<~{tracker.owner.username}/{tracker.name}/{ticket.scoped_id}@example.org>'
+    )
 
     unassign(ticket, assignee1, assigner)
     db.session.commit()

tests/test_comments.py

@@ -45,6 +45,9 @@ def test_ticket_comment(mailbox):
             assert e.headers['From'].startswith(submitter.name)
             if starts_with:
                 assert e.body.startswith(starts_with)
+            assert e.headers["In-Reply-To"] == (
+                f'<~{tracker.owner.username}/{tracker.name}/{ticket.scoped_id}@example.org>'
+            )
 
     def assert_event_notifications_created(event):
         assert {en.user.email for en in event.notifications} == {

tests/test_submit.py

@@ -52,6 +52,9 @@ def test_submit_ticket(client, mailbox, submitter_subscribed):
     assert description in email.body
     assert ticket_url(ticket) in email.body
     assert email.headers['From'].startswith(submitter.name)
+    assert email.headers['Message-ID'] == (
+        f'<~{tracker.owner.username}/{tracker.name}/1@example.org>'
+    )
 
     # Check event notification is created for the subscriber
     assert len(subscriber.user.notifications) == 1
@@ -149,3 +152,7 @@ def test_mentions_in_ticket_description(mailbox):
     expected = f"You were mentioned in {ticket.ref()} by {submitter.name}."
     assert p1_email.body.startswith(expected)
     assert p2_email.body.startswith(expected)
+
+    assert p2_email.headers["In-Reply-To"] == (
+        f'<~{tracker.owner.username}/{tracker.name}/{ticket.scoped_id}@example.org>'
+    )

todosrht/tickets.py

@@ -170,6 +170,7 @@ def _send_comment_notification(subscription, ticket,
     subject = "Re: {}: {}".format(ticket.ref(), ticket.title)
     headers = {
         "From": "{} <{}>".format(participant.name, notify_from),
+        "In-Reply-To": f"<{ticket.ref(email=True)}@{posting_domain}>",
         "Reply-To": f"{ticket.ref()} <{ticket.ref(email=True)}@{posting_domain}>",
         "Sender": smtp_user,
     }
@@ -231,6 +232,7 @@ def _send_mention_notification(sub, submitter, text, ticket, comment=None):
     subject = "{}: {}".format(ticket.ref(), ticket.title)
     headers = {
         "From": "{} <{}>".format(submitter.name, notify_from),
+        "In-Reply-To": f"<{ticket.ref(email=True)}@{posting_domain}>",
         "Reply-To": f"{ticket.ref()} <{ticket.ref(email=True)}@{posting_domain}>",
         "Sender": smtp_user,
     }
@@ -350,6 +352,7 @@ def notify_assignee(subscription, ticket, assigner, assignee):
     subject = "{}: {}".format(ticket.ref(), ticket.title)
     headers = {
         "From": "~{} <{}>".format(assigner.username, notify_from),
+        "In-Reply-To": f"<{ticket.ref(email=True)}@{posting_domain}>",
         "Reply-To": f"{ticket.ref()} <{ticket.ref(email=True)}@{posting_domain}>",
         "Sender": smtp_user,
     }
@@ -434,6 +437,7 @@ def _send_new_ticket_notification(subscription, ticket):
     subject = f"{ticket.ref()}: {ticket.title}"
     headers = {
         "From": "{} <{}>".format(ticket.submitter.name, notify_from),
+        "Message-ID": f"<{ticket.ref(email=True)}@{posting_domain}>",
         "Reply-To": f"{ticket.ref()} <{ticket.ref(email=True)}@{posting_domain}>",
         "Sender": smtp_user,
     }